Zooplankton of the Okavango Delta and associated basins in Botswana

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English: The Okavango Delta, one of the most iconic wetlands on the planet, is the third largest Ramsar site and in 2014 was listed as the 1000th UNESCO World Heritage Site. It is situated in the centre of the Kalahari Basin of southern Africa, which is a semi-desert and the largest continuous sea of sand on earth. The Okavango Delta, in northern Botswana, receives its waters from the Okavango River which originates as many tributaries in Angola and flows as a single river through Namibia’s Caprivi Strip before entering Botswana where it is met by a series of fault lines. These faults control the shape of the delta and result in its water spreading across the Kalahari Sands, forming the world’s largest alluvial fan. Most years there is little to no outflow from the Okavango Delta, but the system recently experienced consecutive years of record high floods from the catchment area in Angola, resulting in the basins associated with the delta being inundated for the first time in up to four decades. This provided the perfect opportunity to study the zooplankton of the Okavango Delta, from which only a few studies have been conducted, and it has allowed an insight into the zooplankton species diversity of the associated basins. The aims of the present study were to obtain specimens of aquatic micro-invertebrates, correctly identify Rotifera and micro-Crustacea material collected up to species level where possible, create a photographic data base of the organisms and to map the distribution of each species throughout the study area. Water bodies included in the study are the Okavango Panhandle, the Okavango Delta Fan, the Thamalakane River, Lake Ngami, the Boteti River and Lake Xau. A total of 33 rotifer species were identified, including three genera (Asplanchnopus de Guerne, 1888, Synchaeta Ehrenberg, 1832 and Sinantherina Bory de St. Vincent, 1826) and six species, i.e. Asplanchnopus multiceps (Schrank, 1793), Epiphanes senta (Müller, 1773), Synchaeta pectinata Ehrenberg, 1832, Conochilus dossuarius Hudson, 1875, Conochilus unicornis Rousselet, 1892 and Sinantherina sp., which have not been identified from the waterways of northern Botswana before. The cladoceran species diversity, excluding the family Chydoridae which was the most diverse, but lacks taxonomic review, totalled 12 species. Amongst these was the species Macrothrix propinqua Sars, 1909 which has not been recorded from the study area previously. The calanoid copepod Tropodiaptomus schmeili (Kiefer, 1926) was present in the Okavango Delta and its associated basins, except in Lake Xau where no calanoids were collected. Furthermore, a possibly undescribed species of Metadiaptomus Methuen, 1910 was found in the Okavango Panhandle and Lake Ngami. Eight species of the order Cyclopoida were collected from the Okavango Delta and its associated basins, five of which were recorded from the system for the first time. Zooplankton studies were also conducted in the Nata River during low flow and results indicated a total of 14 rotifer, cladoceran and copepod species. Unlike the Okavango System where the calanoid genus Tropodiaptomus thrives, Metadiaptomus transvaalensis Methuen, 1910 was found in the Nata River. Furthermore, this calanoid copepod was infested with the peritrich, Trichodina diaptomi Šrámek-Hušek, 1953, of the order Mobilida. Notes on the possible life-history of this trichodinid are provided. The results from the entire study have laid down the framework for future zooplankton research in the waterways of the semi-arid Botswana, where no baseline data exists for many of the life-sustaining water bodies.